Holographic recording that records information in a recording medium using holography is generally performed by an information light that carries image information and a reference light being superimposed within the recording medium. An interference fringe pattern formed at this time is written in the recording medium. When the recorded information is reproduced, the reference light is irradiated onto the recording medium. Image information is reproduced by diffraction caused by the interference fringe pattern.
In recent years, in holographic recording, volume holography, particularly digital volume holography that has been developed for practical use to achieve ultra-high data density is receiving attention. The volume holography is a method in which interference fringes are three-dimensionally written through active use of a thickness direction of the recording medium. While using the same recording medium and recording method as the volume holography, the digital volume holography is a computer-oriented holographic recording method that limits the image information to be recorded to a binarized digital pattern. In the digital holography, even image information such as an analog illustration is digitalized and developed into two-dimensional digital pattern information. The two-dimensional digital pattern information is then recorded as the image information. During reproduction, the digital pattern information is read and decoded, thereby returning the digital pattern information to the original image information. The image information is then displayed. As a result, even when a signal-to-noise (SNS) ratio is somewhat poor during reproduction, the original information can be very accurately reproduced through differential detection and through binary data encoding and error correction.
An example of recording being performed in a hologram recording layer by volume holography is as follows. An information light carrying information to be recorded and a recording reference light are simultaneously irradiated from a transparent substrate side for a predetermined amount of time such that interference fringes in a thickness direction are formed within the hologram recording layer. An interference fringe pattern is three-dimensionally fixed within the hologram recording layer. As a result, the information is recorded as a three-dimensional hologram (Patent Literature 1 and Patent Literature 2).
Optical information recording methods include a method in which recording is performed with the information light and the recording reference light being disposed on optical axes of differing angles and a method in which recording is performed with the information light and the recording reference light being disposed on a same axis. The recording method in which the information light and the recording reference light are disposed on the same axis is receiving attention as a method suitable for volume holography, such as that described above (refer to, for example, Patent Literature 3).    Patent Literature 1: Japanese Patent Laid-open Publication No. Heisei 11-311938    Patent Literature 2: Japanese Patent Laid-open Publication No. 2003-99952    Patent Literature 3: Japanese Patent Laid-open Publication No. Heisei 10-124872